A thermal interface material (TIM) used for dissipating heat to a heat sink or heat spreader preferably has a high intrinsic thermal conductivity. In addition, it is desirable that the TIM allows for high thermal conductivity at its contact with other materials. An effective TIM is typically mechanically compliant in order to optimize the area of contact. Examples of some known thermal interface materials include thermal greases, epoxy adhesives, and thermal gels such as silicones or olefins.
Carbon nanotube (CNT) composite films comprising aligned arrays of nanofibers have extremely high intrinsic thermal conductivity, and are thus attractive candidates for a TIM. For example, US Pat. Pub. 2005/0171269 describes a composite CNT TIM containing randomly oriented CNTs in a matrix material such as thermal grease. The thermal conductivity of such designs, however, is limited by the random orientation of the CNTs in the matrix. Accordingly, other CNT composites have used aligned CNTs vertically grown in arrays. These arrays may be made with interstitial filling material (e.g., polymer or metal), such as described in US Pat. Pub. 2003/0117770, US Pat. Pub. 2005/0224220, and US Pat. Pub. 2006/0073332. However, such materials can have a large thermal resistance at their contact interface with other materials, severely reducing the net thermal performance of a potential CNT composite TIM.
Some known techniques for reducing surface contact resistance include topography matching (e.g., polishing both surfaces to be flat) as in US Pat. Pub. 2003/0117770, the use of thermal grease as in US Pat. Pub. 2005/0150887, and the use of compliant thermal interface materials such as Microfaze A6 which is a gel sandwiching an aluminum film. In US Pat. Pub. 2004/0009353 Knowles describes techniques for reducing surface contact resistance including the use of phase change materials such as Hi-Flow 225U which is a wax with embedded thermally conductive particles, the application of heat and pressure to the TIM, and the use of nanofibril “whiskers”. Knowles discloses an aligned nanofiber array sandwiched between two sheets of phase change material which may be supported by release liner paper which can be peeled away before application, e.g., like a “tape.”
Although the teachings of Knowles overcomes some of the problems in the art, the binder is limited to wax-based materials, and the techniques require synthesis of an interface material on the two surfaces to be put into thermal contact by the TIM. In addition, existing TIMs still suffer from poor thermal contact between the nanofibers and the substrate. Thus, it would be desirable to overcome various remaining limitations in the current state of the art by providing more versatile and better performing TIMs based on novel materials, structures, and methods.